What Does DSLR Stand For?
DSLR stands for ddigital single-lens reflex. Sound complicated? Well, DSLRs use a series of mirrors to reflect light through a single prism, which then refracts the light into the light sensors. That's it!
Speaking old school photography here, one would typically use the viewfinder to, well, view the image. In DSLRs, the light comes through the camera via a series of lenses, including a “4 elements lens" that includes two convex and two concave lenses. This is then reflected by the “reflex mirror"--where DSLRs get their name—across a series of mirrors into the viewfinder and the eye of the photographer.
To take a picture, the aperture size either increases or decreases according to the desired setting and the same reflex mirror slides out of the way of the light as the shutter opens. The image sensors then take in light for the duration of the exposure. This explains the “viewfinder blackout" that one gets as one takes the picture while looking through the viewfinder as the mirror flips up and no longer reflects light into your eye. Most of the time lapse in taking a photo is getting all those mirrors and apertures to move more quickly, because even milliseconds is too much of a drag for many action photographers.
Of course, there are just plain old SLRs without the digital "D" - these are film cameras. It is for these that viewfinders were first designed as we know them in DSLRs, because alternatives like LCD screens didn't exist until quite recently. However, these are not the focus of the article.
There are other features that are associated with DSLRs, even though they aren't a part of the technical definition: